Moberg et al: Highly variable Northern Hemisphere temperatures?

The 10th Feb edition of Nature has a nice paper “Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data” by Anders Moberg, DM. Sonechkin, K Holmgren, NM Datsenko, & W Karlin (doi:10.1038/nature03265). This paper takes a novel approach to the problem of reconstructing past temperatures from paleoclimate proxy data. A key result is a reconstruction showing more century-scale variability in mean Northern Hemisphere temperatures than is shown in previous reconstructions. This result will undoubtedly lead to much discussion and further debate over the validity of previous work. The result, though, does not fundamentally change one of the most discussed aspects of that previous work: temperatures since 1990 still appear to be the warmest in the last 2000 years.

The novel thing about this paper is the use of wavelets (a statistical tool common in image-processing software) to separate the low and high-frequency components of the data. The temperature reconstruction is then formed by combining the high-frequency (< 80 y) signal from tree rings with the low-frequency (> 80 y) signal from lake sediments and other such non-annually resolved proxies. This does two things that may be important: it allows the non-annually resolved proxies to be used (recent previous reconstructions, e.g. MBH98, Esper et al., used only those with at least a value each year, to allow calibration against the instrumental record; Moberg’s approach allows the use of data that only provide 50 y means); and it throws away the long-term signal from the tree rings, which they consider to be untrustworthy. Other techniques have also been used or suggested for the merging of the low and high frequency signals (Guiot, 1985; Rutherford et al. (2005)).

The result is a more long-term variable signal than, e.g., Mann, Bradley and Hughes (1998; henceforth MBH98). Moberg et al. end up with two “warm peaks” in the smoothed record around 1000 and 1100 A.D., at 0 ºC on their anomaly scale. A few individual years within these intervals are almost +0.4 ºC warmer than the average. In comparison, the most recent data from the instrumental record post 1990 peak at +0.6 or a bit more, on the same scale. The coldest years of the so called “Little Ice age” occur around 1600 and are about -0.7 colder than average, with individual years down to -1.2 ºC.